In 2050, Haesslich began a romantic relationship with the human Nadia Mirin (birth name Dawn McGrath), a powerful mage and executive at the Natural Vat corporation. He was commonly seen with her in human form (under the assumed name of Jarlath Drake), rescued her from an assassination attempt, and battled the corrupt Lone Star security forces who had attempted her assassination, destroying an attack helicopter in dragon form. 
Despite this, Haesslich was not known for his altruism. Later in that same year, he hired under personal contract Katherine Hart and her partner feathered serpent, Tessien, to do some work for him involving the secret planting of a doppelganger in the Renraku Arcology.
The doppelganger was created by employing both science and magic, with help from a scientist in the employ of Lofwyr. It was created to infiltrate the Renraku Arcology and steal information about the project they were working on: the creation of a fully sentient AI.
His plot is ultimately unraveled in a series of betrayals and double crossings by and to Katherine Hart, Sam Verner, Alice Crenshaw, and Lofwyr (via the sasquatch Jaquline), resulting in Haesslich killing Tessien, then attempting to kill Verner.
Haesslich was shot from the sky by Verner's shadowrunners and fell into Puget Sound. He was not seen again and presumed dead. 
"Haesslich" is German for "ugly," a name possibly taken ironically, as both Haesslich's human and dragon forms were considered particularly beautiful. Though (presumably) not a Great Dragon, he was able to take human form using a magical item, a golden bracelet in the shape of a dragon.
Official sources state that Haesslich is a Great Dragon (Never Deal with a Dragon and Sixth World Almanac). However, the idea that a team of runners are able to actually kill a Great Dragon with a minigun doesn't make much sense, nor would it make sense for a Great Dragon to be a lowly employee (of a non-AAA corporation, even) or to need a magical item in order to shapeshift. There are two theories on how to reconcile this discrepancy. Ether Haesslich is a Great Dragon and faked his own death (possibly to get Lofwyr off his back), and is now lying low and plotting his next move from the shadows for the past few decades. Or that Haesslich is a narcissist that thought of himself as a Great and was actually killed by Twist and company. His body was recovered by United Oil from Puget Sound according to New Seattle (p.88), however no one has seen it since then, and no confirmation if he was dead or alive have been reported.